Introduction
The p-block contains several elements of great social and economic importance as well as chemical interest. Examples include the use of aluminium as a structural material, the importance of silicon and germanium as semiconductors, and the use of sulphur, phosphorus and nitrogen in fertilizers.
Group 13 Elements (Boron Family) - Introduction
Group 13 of the periodic table consists of the elements boron (B), aluminium (Al), gallium (Ga), indium (In) and thallium (Tl). Except boron, which is classified as a non-metal, all other elements of this group are metals.
Occurrence and Uses
Aluminium is the third most abundant element (8.3% by weight) in the earth's crust after oxygen (45.5%) and silicon (25.7%).
Physical Characteristics of Group 13 Elements
The important physical constants of group 13 elements arc given in table and their physical characteristics are discussed below:
| B | Al | Ga | In | Tl | |
| Atomic radius (pm) | 85 | 143 | 135 | 167 | 170 |
| Ionic radius (pm) M3+ | 27 | 53.5 | 62.0 | 80.0 | 88.5 |
| Density (gcm -3) | 2.35 | 2.70 | 5.90 | 7.31 | 11.85 |
| Ionization wnergy I KJ mol -1 II III | 800 | 577 | 578 | 558 | 590 |
| 2429 | 1816 | 1979 | 1820 | 1971 | |
| 36.59 | 2744 | 2962 | 1704 | 2877 | |
| Electronegativity | 2.0 | 1.5 | 1.6 | 1.7 | 1.8 |
| Melting point (K) | 2453 | 933 | 303 | 430 | 576 |
| Boiling point (K) | 3923 | 2740 | 2676 | 2353 | 1730 |
Trends in Chemical Reactivity of Group 13 Elements
Unlike boron, the other members of the group are metallic and the electropositive character increases as we go down the group. All these elements in their compounds exhibit the oxidation state of + 3 and +1.
Anomalous Behavior of Boron and Resemblance with Silicon
Because of extremely small size of its atom and high electronegativity, boron differs in its properties from other elements of the group. On the other hand, it resembles silicon to some extent due to diagonal relationship. The following properties illustrate the anomalous behavior of boron and its diagonal relationship with silicon.
Extraction of Aluminium
Pure Aluminium metal is extracted from bauxite in a three-stage process.
a) Purification of Bauxite (Bayers Process) to obtain pure Alumina.
b) Electrolysis of pure Alumina in molten cryolite (Na3AlF6) (Halls process).
c) Refining of Aluminium (Hoopes process).
Properties and Uses of Aluminium
Aluminium is a light silvery-white metal with high tensile strength, a high electrical and thermal conductivity. On a weight-to-weight basis, the electrical conductivity of aluminium is twice that of copper.
Group 14 Elements (Carbon Family) - Introduction
Carbon (C), silicon (Si), germanium (Ge), tin (Sn) and lead (Pb) constitute the group 14 of the periodic table. This group is known as carbon family.
Atomic Properties - Electronic Configurations
The elements of group 14 have four electrons in the outermost orbitals. The general configuration may be written as ns2np2 (one electron in the outermost p-orbitals and two electrons in the s-orbital).
Occurrence and Uses
Silicon is present in nature in the form of silica (SiO2) and silicates. Rocks and clays consist of silicate minerals. Silicon is the second most abundant element (~ 27.2 wt%) on the earth's crust after oxygen {45.5wt%}. Germanium is a trace element (1.5 ppm) and is mainly recovered from flue dusts arising from the roast1ng of zinc ores. The natural abundances of tin and lead are 2 ppm and 13 ppm respectively. Tin occurs mainly as cassiterite, SnO2. The principal ore of galena (PbS) is often found in association with zinc blende (ZnS). Other ores of lead are anglesite, (PbSO4) and cerussite (PbCO3). Small quantities of lead ores occur in Rajasthan. Silicon and germanium (though to a lesser extent) are used in the production of semiconductors and integrated circuits. Silicon is a very important component of ceramics, glass and cement. Ge is transparent in the infrared region and therefore is used in the making of infrared windows, prisms and lenses. Tin is used as a coating on metals and in making various alloys like solder, bronze and type metal. Lead is mostly used in storage batteries, in alloy making and pigments/ chemicals.
General Characteristics of Group 14 Elements
| C | Si | Ge | Sn | Pb | |
| Atomic radius (pm) | 77 | 118 | 122 | 140 | 146 |
| Ionic radius (pm) | - | 40 | 53 | 69 | 78 |
| M4+ M2+ | - | - | 73 | 118 | 119 |
Melting Points and Boiling Points
The melting and boiling points of group 14 elements are much higher as compared to those of group 13 elements.
Trends in Chemical Reactivity
Some important group trends are:
(a) On account of high ionization enthalpies, simple M4+ ions of the group are not known.
(b) Unlike carbon the other elements of the group form compounds having coordination numbers higher than 4 like (SiF5)-, (SiF6)2- and (PbCl6)2-.
Forms of Silica
Silica is widely distributed in nature as sand. It is found in nature in many different forms:
i) Quartz or rock crystal
ii) Sand
iii) Artificial silica
Structure of Silica
A study of crystal structure of silica reveals that it forms a giant molecule as shown below.
Uses of Silica
i) Quartz glass is used for manufacturing optical instruments. Under the trade name Vitreosil.
ii) Colored quartz is used for manufacturing gems.
Silicates
Silicate is the general term given to solids with silicon-oxygen bonds. A large percentage of the earth's crust consists of silicate minerals. Some of the important silicate minerals are quartz, asbestos (calcium magnesium silicate, CaMgSi2O6), feldspar (potassium aluminium silicate, KAlSi3O8) and zeolites (sodium aluminium silicate, Na2Al2Si2O8.xH2O).
Silicones
Silicones are polymeric compounds containing Si-O-Si linkages. These have the general formula (R2SiO)n. These may be linear cyclic or cross-linked. These have very high thermal stability and are called high temperature polymers (R may be alkyl or phenyl group). The starting material for the manufacture of silicones is alkyl substituted chlorosilanes.
Zeolites
Zeolites are microporous aluminosilicates of general formula Mx/n[AlO2]x[SiO2].mH2O and may be considered as open structures of silica in which aluminium has been substituted in a fraction x / (x+y) of the tetrahedral sites.
Glass
Ordinary glass is a mixture of sodium and calcium silicates and is made by heating a mixture of sand (essentially SiO2) with sodium carbonate and calcium oxide in a furnace at around 1700K and cooling rapidly.
Tin and Lead
Tin and lead (Sn and Pb) belong to group 14 of the periodic table. They have four electrons in their valence shell, their valence shell configuration being ns2 np2. These elements exhibit oxidation states of +2 and +4. The oxidation state +2 is more common because the s-electrons do not participate in bonding easily due to inert pair effect.
Extraction of Tin and Lead
The chief ore of tin is cassiterite or tinstone, SnO2. This ore contains about 10% of tin. Tin is extracted from cassiterite through the following steps:
1. The crushed ore is washed with water to remove lighter impurities.
2. The ore is then roasted to remove arsenic and sulphur as volatile oxides.
3. Tin is obtained by reducing it with coal in a reverberatory furnace; Limestone is added to produce a slag with the impurities, which can be removed.
Uses of Tin and Lead
1.Tin is used to tin plate iron, which is then used to make the so called 'tins' for canning fruit, jam, pickles, etc.
2. It is also used for tinning copper and brass utensils.
3. It is used to make certain alloys such as bronze, bell metal, etc., which have specific uses.
Compounds of Tin and Lead
Tin and lead form both divalent, i.e., Sn (II), and lead (II), and tetravalent, i.e. tin (IV) and lead (IV) compounds.
Group 15 Elements (Nitrogen Family) - Introduction
Group 15 of the Periodic table includes five elements namely nitrogen (N), phosphorus (P), arsenic (As) antimony (Sb) and bismuth (Bi). This group is regarded as nitrogen family. There is a transition from non-metallic to metallic character as we go down the group. Nitrogen and phosphorus are non-metals: arsenic and antimony are semi-metals or metalloids; bismuth is metallic. Besides nitrogen, the other important element of group 15 is phosphorus.
Occurrence and Uses of Phosphorus
Phosphorus is widely distributed in nature. It does not occur in free state since it can be very easily oxidised. It is usually present in rocks in the combined state as phosphates.
Atomic Properties - Electronic Configurations
The atoms of group 15 have five electrons in the outermost shell, two in s and three in p sub-shell. The general electronic configuration of this group may be expressed as ns2np3.
Physical Characteristics of Group 15 Elements
| N | P | As | Sb | Bi | |
| Atomic radius (pm) | 70 | 110 | 120 | 140 | 150 |
| Ionic radius (pm) | 171 (N3-) | 212 (P3 - ) | 222(As3 - ) | 76(Sb3 + ) | |
| Ionoization energy IE1 (KJ mol - 1) | 1402 | 1012 | 947 | 834 | 703 |
Chemical Properties
Nitrogen is a colorless gas and exists as diatomic. The two nitrogen atoms are held together by triple bond and have very high bond dissociation energy (945 kJ mol-1).
Hydrides
The elements of groups 15 form trihydrides of the general formula MH3 such as
| NH3 | PH3 | AsH3 | SbH3 | BiH3 |
| Ammonia | Phosphine | Arsine | Stibine | Bismuthine |
Oxides
The elements of group 15 combine with oxygen either directly or indirectly to form oxides. The important oxides of group 15 elements along with their oxidation states are listed in table.
Halides
Group 15 elements form two series of halides of the type MX3, (trihalides) and MX5, (pentahalides). The trihalides are formed by all the elements while pentahalides are formed by all the elements except nitrogen. Nitrogen cannot form pentahalides due to the absence of vacant d-orbitals in its outermost shell. Similarly the last element, Bi has little tendency to form pentahalides because +5 oxidation state of Bi is less stable than +3 oxidation state due to inert pair effect.
Allotropes of Phosphorus
Elemental phosphorus is obtained by heating phosphate rock with coke and silica in an electric furnace at about 1770 K. The phosphorus so formed is white phosphorus.
Phosphine
Preparation : (i) From Phosphide, (ii) From Phosphorus acid.
Oxoacids
The fertility of soil can be enhanced by using chemical fertilizers, which provide the essential plant nutrients, potassium, nitrogen and phosphorus.
Phosphate Esters
Phosphate esters play a vital role in the life processes. The most important of these biomolecules are DNA, RNA, adenosine mono-, di- and tri- phosphates (AMP, ADP and ATP). Hydrolysis of P-O-P link in ADP releases energy, which can be put to useful work.
Group 16 Elements (Chalcogens) - Introduction
Oxygen, sulphur, selenium, tellurium and polonium constitute the group 16 of the periodic table of elements. The first four elements are collectively called CHALCOGENS. The name derives from the Greek word for bronze and points to the association of sulphur and its congeners with copper. All these are non-metallic in character. The metallic character increases down the group. The last element of the family, polonium has metallic character and is a radioactive element with a very short life period. Polonium derives its name from Poland, the home country of Marie Curie who discovered the element in 1898.
Occurrence and Extraction
Sulphur occurs in the earth's crust to the extent of 0.05% mostly as metal sulphides and sulphates. It also occurs in the elemental form in large underground beds. Another major source of sulphur is H2S present in the natural gas and crude oil. Sulphur exists in combined state in living matter and is a constituent of some amino acids (e.g.. cysteine), proteins and enzymes.
Extraction of Sulphur
Sulphur is extracted from underground deposits by pumping superheated steam (-440 K) down the beds to melt the element and then blowing out the molten sulphur with compressed air. This process is known as Frasch process.
Uses of Sulphur, Selenium and Tellurium
(i) Most of the sulphur produced is used for the manufacture of sulphuric acid and other industrially important sulphur compounds.
(ii) Sulphur is used to prepare large number of medicines (Sulpha drugs) and can be used as ointment for curing skin diseases.
Atomic Properties
The elements of this group have six electrons in the outermost shell and have the general electronic configuration as ns2np4.
Physical Characteristics Group 16 Elements
| Property | Oxygen | Sulphur | Selenium | Tellurium | Polonium |
| Atomic radius (Ao) | 0.73 | 1.09 | 1.16 | 1.35 | - |
| Ionic (M2+) radius (Ao) | 1.40 | 1.85 | 1.98 | 2.21 | |
| Ionization energy (kJ mol -1) | 1314 | 1000 | 941 | 869 | - |
Oxidation States
All the elements of this group have ns2np4 configuration in their outermost shell. Therefore, the atoms of these elements try to gain or share two electrons to achieve noble gas configuration. Since oxygen has a very high value of electronegativity, it tends to achieve noble gas configuration preferably by gaining electrons. Thus, oxygen exhibits oxidation state of -2 in its compounds. However, there are a few exceptions in which the oxidation state of oxygen is not -2. For e.g., the oxidation state of oxygen in F2O is + 2, while in H2O it is -1.
Trends in Chemical Reactivity
As in the case of S, the important oxidation states observed for Se and Te are also 2, +2, +4 and +6. The chalcogenide dianions (E2-) exist only in their compounds with the most electropositive elements. By and large, chalcogens with the exception of polonium, display non-metallic covalent chemistry.
Chemical Properties - Hydrides
Both oxygen and sulphur are very reactive and the reactivity decreases as O > S > Se > Te. Oxygen is very reactive and reacts with almost all elements. Its reactivity is only slightly less than the most reactive elements, halogens.
Trends in Characteristics
All the hydrides are volatile. The volatility increases from H2O to H2S and then decreases. This is clear from their boiling points as given below:
| H2O | H2S | H2Se | H2Te |
| 373 K | 213 K | 232 K | 269 K |
Halides
Since fluorine is more electronegative than oxygen, its compounds with oxygen are called fluorides. For e.g., F2O is written as OF2 and is named as oxygen difluoride. However, chlorine, bromine and iodine are less electronegative than oxygen, therefore, the compounds of these halogens with oxygen are called oxides. For e.g., ClO2 is named as chlorine dioxide, I2O5 is named as iodine pentoxide and Cl2O7 is named as chlorine heptoxide.
Oxides
The elements of group 16 form a variety of oxides. Some of these are given below:
| Element | Oxides | |||
| S | SO | SO2 | SO3 | S2O7 |
| Se | - | SeO2 | SeO3 | - |
| Te | TeO | TeO2 | TeO3 | - |
| Po | PoO | PoO2 | PoO3 | - |
Oxoacids
Among the members of oxygen family, sulphur forms a number of oxoacids. The structures along with the oxidation state of some important oxoacids of sulphur are shown in below figures.
Anomalous Behavior of Oxygen
Like all other first elements of the groups, oxygen also differs from its family members. The anomalous behavior of oxygen is due to its
(i) small size
(ii) high value of ionization energy and electronegativity
(iii) absence of d-orbitals in the valence shell.
Sulphur Production and Allotropes
Sulphur is the second member of the oxygen family, yet it differs from oxygen in many respects. For e.g., O2 exists as molecule while S2 molecule does not.
Oxides of Sulphur
Two of the important oxides of sulphur are sulphur dioxide and sulphur trioxide. Sulphur dioxide is formed by burning sulphur in air or roasting metal sulphides in the presence of air.
Sulphuric Acid
It is the most important oxoacid of sulphur and is known from ancient times. It is called 'oil of vitriol' because in early days it has been prepared from ferrous sulphate crystals (green vitriol) and has an oily appearance. It occurs in certain minerals and springs. Because of its large applications in industries it is also known as the "King of chemicals".
Group 17 Elements (The Halogen Family) - Introduction
Group 17 of the periodic table contains the elements fluorine, chlorine, bromine, iodine and astatine. These elements are collectively known as the HALOGENS. This name is derived from two Greek words Halo and Gens meaning 'salt producing'. Halogens are among the most reactive non-metals. There are greater similarities within the halogen group compared to other groups in the periodic table with the exception of the alkali metals (group I). Astatine is radioactive and is therefore not of any practical importance. All halogens precede the noble gases in the periodic table because their atoms contain one electron less than the electronic configuration of the inert gas. They have very strong tendency to acquire stable inert gas configuration by accepting one electron. For this reason halogens exhibit non-metallic behavior.
Occurrence
Astatine the last member of halogen family is a radioactive element with a very short life. All other halogens are abundant in nature in the combined state. The very high electronegativities of halogens make them very reactive and hence, they are not found in free state.
Uses - Fluorine
1. It is mainly used for the manufacture of UF6 for nuclear power generation.
2. It is also used for the preparation of many fluorinating agents. The important organic chemicals derived from HF are the chlorofluorocarbons and polytetrafluoroethylene (teflon). Chlorofluorocarbons known as freons are used as refrigerants and in aerosols. Teflon is a plastic, which is not attacked by chemical reagents and is heat-resistant. It is used for many special applications.
3. An important inorganic chemical made from HF is cryolite (Na3AlF6) used for the production of aluminium. Some of the uses of HF are in the glass industry as an etching agent and in the manufacture of fluoride salts. Prominent among the fluorides is NaF used for the fluorination of water; one part per million level fluoride in drinking water prevents tooth decay.
Atomic Properties - Electronic Configurations
The atoms of group 17 have seven electrons in the outermost shell, and have the general electronic configuration ns2np5.
Physical Characteristics of Group 17 Elements
Atomic and ionic radii
The halogens have the smallest atomic radii in their respective periods due to maximum effective nuclear charge. Among themselves, the atomic and ionic radii increase with increase in atomic number. This is due to increase in the number of electron shells.
Oxidation States
Halogens have only one electron less than the next noble gas. Therefore, they can get the noble gas configuration either by gaining one electron to form uninegative ion, X-, or by sharing electrons with other atoms. Thus, they show an oxidation of state of l or + 1. Since fluorine is the most electronegative element, it always shows an oxidation state of -1. It does not show any positive oxidation state.
Trends in Chemical Reactivity of Elements and Compounds
The halogens are the most reactive elements as a family. Fluorine is the most reactive of all the halogens. The reactivity of the halogens decreases down the group.
Oxidising Power of Halogens
Halogens have high electron affinity values and therefore they have strong tendency to take up the electron:
Hydrides
All the halogens react with hydrogen to form hydrides, HX known as halogen hydrides.
Oxides of Halogens
Fluorine forms two oxides F2O and F2O2, which are called oxygen fluorides. In this case fluorine is more electronegative than oxygen. On the other hand the oxides of Cl, Br and I are called oxides. They form oxides from + 1 to + 7 oxidation states.
Oxoacids of Halogens
The oxoacids of halogens are given in below shown table. It may be noted that fluorine does not form oxoacids because fluorine is more electronegative than oxygen.
Bleaching Powder
Bleaching powder is also called calcium chlorohypochlorite because it is considered as a mixed salt of hydrochloric acid and hypochlorous acid. It is represented as:
Interhalogen Compounds
The compounds containing two or more halogen atoms are called inter halogen compounds. Each halogen combines with every other halogen to form interhalogen compounds. For e.g., ClF, ICl3, BrF5 etc.
Group 18 Elements (Noble Gases) - Introduction
The elements helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn) are grouped together in Group 18 of the Periodic Table. All of these are gaseous under ordinary conditions of temperature and pressure. The last number of the group, radon is obtained from radioactive disintegration of radium. All others are present in air in traces. They are also known as rare gases because they are found in very small quantities in nature. They are highly non-reactive and do not take part in chemical combinations to form compounds like other elements and are, therefore, also called inert gases.
Discovery of Noble Gases
Discovery of noble gases is an interesting story, which reveals painstaking work of different scientists. i) Discovery of argon, ii) Discovery of helium, iii) Discovery of xenon, krypton and neon, iv) Discovery of radon.
Occurrence
Noble gases always occur in free state because of their inert nature. All the noble gases, except radon are present in air in small amounts. The relative abundance of the noble gases in air is = 1%. Helium is present in natural gas up to the extent of 10 per cent. It is also present in small quantities in the minerals of radioactive elements. Water from certain springs also gives off gases, which are rich in helium and argon.
Atomic Properties Electronic Configuration
| Element | Symbol | Atomic No. | Electronic Configuration | Abundance in Earth's crust (in ppm) |
|---|---|---|---|---|
| Helium | He | 2 | 1s2 | 5.2 x 10 - 4 |
| Neon | Ne | 10 | [He] 2s22p6 | 1.8 x 10 - 3 |
| Argon | Ar | 18 | [Ne] 3s23p6 | 9.3 x 10 - 1 |
| Krypton | Kr | 36 | [Ar] 3d104s24p6 | 1.1 x 10 - 4 |
| Xenon | Xe | 54 | [Kr] 4d105s25p6 | 8.7 x 10 - 6 |
| Radon | Rn | 86 | [Xe] 4f14 5d10 6s2 6p6 | - |
Isolation Of Noble Gases
Helium
It is commercially obtained from natural gas, which contains H2S, CO2, H2O, N2, hydrocarbons and helium. Natural gas is compressed and cooled when nitrogen, hydrocarbon etc. get liquefied while helium remains unliquified, as a gas, which is purified.
Uses of Noble Gases
Argon is used mainly to provide an inert atmosphere in high temperature metallurgical processes (arc welding of metals or alloys) and for filling electric bulbs. It is also used in the laboratory for handling substances that are air-sensitive.
Physical Characteristics of Group 18 Elements
Their physical properties show a regular and smooth gradation.
Chemical Properties
The noble gases are generally inert and do not participate in the reactions easily. The inertness of noble gases is due to the following reasons.
i) The atoms of noble gases have stable closed shell electronic configuration.
ii) The noble gases have exceptionally high ionization energies.
iii) The noble gases have very low electron affinities.
Compounds Of Xenon - Xenon Fluorides
Xenon forms three fluorides, XeF2, XeF4 and XeF6. These can be obtained by the direct interaction between xenon and fluorine under appropriate experimental conditions,
Key Concepts
It refers to the similarly in the properties of some of the elements of 2nd period to diagonally opposite elements of 3rd period. The pair of elements showing diagonal resemblance are: Li……Mg, Be……..Al and B…….Si.
Summary
In group 13, the characteristic group oxidation state of + 3 is known but the + 1 state gets progressively stabilized on going down the group. It is particularly favored by Tl. This is a consequence of the so called 'inert pair effect' which also accounts for the stability of the dipositive oxidation states of Sn and Pb and tripositive state of Bi. The hydrides and halides of Al to Tl are Lewis acids, The basic character of oxides of Al to Tl increases down the group.
